M. Heydari-Malayeri - Paris Observatory

One of the five → Lagrangian points, denoted L1,
which lies between the two bodies on the line passing through their center of mass.
In a → close binary star system
mass transfer occurs through this point.

On of the five locations in space where the → centrifugal force
and the
→ gravitational force of two bodies
(m orbiting M) neutralize each other. A third, less massive body,
located at any one of these points, will be held in equilibrium with
respect to the other two. Three of the points, L1, L2, and L3, lie on
a line joining the centers of M and m.
L1 lies between M and m, near to m, L2 lies beyond m,
and L3 on the other side of M beyond the orbit. The other
two points, L4 and L5, which are the most stable, lie on either side
of this line, in the orbit of m around M, each of them making an
equilateral triangle with M and m.
L4 lies in the m's orbit approximately 60° ahead of it,
while L5 lies in the m's orbit approximately 60° behind m.
See also → Trojan asteroid;
→ Roche lobe;
→ equipotential surface;
→ horseshoe orbit.